When designing microwave circuits, microstrip transmission lines are commonly used. A microstrip transmission line comprises a metal ground plane and a conductor, where a dielectric carrier material is positioned between the metal ground plane and the conductor. This configuration is economical and relatively easy to design.
However, due to losses in the dielectric carrier material, it is sometimes not possible to use microstrip transmission lines. When there for example is a filter in the layout, the filter may have to be realized in waveguide technology. Waveguides normally are filled with air or other low-loss materials.
When there is a filter in a microwave circuit microstrip layout, the filter may thus be realized by means of a waveguide filter in order to lower the losses. In that case, there has to be corresponding microstrip to waveguide transitions at the ends of the filter. Such a waveguide is preferably surface-mounted, enabling it to be mounted to the dielectric carrier material.
Such a surface-mounted waveguide is normally made having three walls and one open side. Metalization is then provided on the side of the dielectric carrier material facing the waveguide, where the metalization serves as the remaining wall of the waveguide, thus closing the waveguide structure when the waveguide is fitted to the dielectric carrier material.
Another application for surface-mounted waveguides is when there has to be a microstrip to waveguide transition in the form of a bend, allowing a waveguide to be mounted to the dielectric carrier material in such a way that it extends essentially perpendicular to the main surfaces of the dielectric carrier material.
It is also conceivable that a waveguide filter is realized having a separate fourth closing wall made as a metalization on a dielectric carrier material, where such a design is found cost-effective.
As disclosed in the paper “Surface-mountable metalized plastic waveguide filter suitable for high volume production” by Thomas J Müller, Wilfried Grabherr and Bernd Adelseck, 33rd European Microwave Conference, Munich 2003, a surface-mountable waveguide is mounted on a so-called footprint, a metalization that generally follows the structure that shall be soldered to it, in this case the contact surface of the waveguide. Furthermore, the footprint comprises the fourth closing wall for the waveguide. Outside the footprint, a so-called solderstop is applied, a type of wall preventing solder from flowing outside the footprint. Solder is dispensed sparsely and self-alignment occurs during soldering.
There is, however, a problem with the design according to the paper, since it still is difficult to control the height of the solder joint. This height is very important to control, since it becomes a part of the waveguide design, and has to be taken into account as a design parameter when designing the waveguide. If this height is inadequately controlled, the waveguide will not function as desired, since the distance between the closing wall and the opposite wall will not be the desired one. This problem of course grows with higher frequency.
There is thus a demand for a surface-mounted waveguide arrangement that provides a more accurate control of the mounted solder height, and a surface-mount method that provides a more accurate control of the mounted solder height. This allows for a very good prediction of the mounted solder height.